Agree but due a potential BC Break I suggest apply it in the next major version

e2d8c14 and 643a99e where BC breaks, this restores the error somewhat.

At the moment the return value of ::run is unusable because you can't know for sure what is returned (note that it still returns the ResponseInterface in some cases before my PR). BC break or not, this is a bug and should be fixed.

The documentation should be updated too, whatever fix we do because this is simply not true no matter how we choose to fix it:

<?php
$application = $serviceManager->get('Application');
$application->bootstrap();
$response = $application->run();
$response->send();

Because a response doesn't have a send method, and the application does not return a response. And depending the routes / application environment this could be a fatal error.

Either we choose to always return Application or ResponseInterface. I can of course send this PR to develop if you wish.

If you tell me your wishes, I can make the changes.

It's a documentation issue at this point. We modified how Zend\Mvc\Application works, as there were issues with simply rendering the response (if you are doing a stream, for instance, an echo was a bad idea). So, the run() method's signature should be updated, and documentation updated. We already updated the skeleton application some time ago, and messaged the change when it was released.

Note that it still returns a resonseinterface in some cases.

@reenl I'll correct hose; they're legacy at best, and the method should be consistent.